Motor driven garage doors of the upward opening and downward closing type have been provided with obstruction detection switches operated by means of a trip bar at the lower leading edge of the door. The switch is arranged so as to stop or reverse the door if the trip bar strikes an obstruction during descent. While such means are effective they add to the complexity of the door manufacture and installation and therefore to cost.
It has been practiced to sense the absolute load of the driven system either by reaction of the drive, for example as described in Australian patent specification No. 521,735, or by change in current flow in the motor circuit. Such systems suffer from nuisance trips due to changes in ambient and local conditions or are maladjusted or desensitized so that nuisance trips do not occur but then the system tends to become a safety hazard. When the door is a rolling door then the motor load changes progressively and non-linearly between open and closed positions as a result of friction and making adjustment of trip level even more difficult. Thus, to date, for rolling doors the only satisfactory method for reversing the door when an obstruction is encountered during descent has been the trip bar previously described.